Help ! Its all greek to me ?

[Mike Cook 3Participant @mikecook3]

Sorry Guys Another question if I may…

it’s been a long time since I did A Level physics, Faradays Law, Ohms Law, Coulombs and all that and in order to make a considered choice of battery, ESC, motor combination, potential run times etc. can anyone explain in simplest terms the relationship between motor kv, load current, ampage, milliamp hours etc..as it’s a long and distant memory and its all Greek to me now. !

Many thanks

[Mike Cook 3Participant @mikecook3]

[Dave MilbournParticipant @davemilbourn48782]

Motor KV = rating of motors (usually brushless) in terms of speed per volt applied. Don't quite know where the K came from; it usually indicates a factor of 1000, but motors are rated in full terms e.g. a motor of 1100 KV will turn at 1100 revs per minute for each 1 volt supplied (so it will do 4070 RPM from a 3.7v LiPo cell)

Load current = current drawn under load e.g. for a motor this will be the current drawn from the battery with the motor running a propeller in the water.

Miiliamp Hour = unit of battery capacity. A battery with a capacity of 2300 Milliamp.Hours (maH) will supply an average current of 2300 milliamps (or 2.3A) for one hour.

Ampage = Should be Amperage and is another word for current.

Etc = Latin words "et cetera" meaning "and other things". I don't think the Greeks had a single word for it either.

Suggest you get hold of a copy of the most recent Model Boats Winter Special. There is a long article in there covering all of this stuff in much more detail.

Dave M

[Mike Cook 3Participant @mikecook3]

Thanks Dave..

Could I be cheeky and ask what you would install in my 46 in Huntsman.( Motor, ESC, Power supply) A recommendation from the original designer would be well received.

I have budgeted about £150 to complete the installation.

Many thanks.

Mike

[Mike Cook 3Participant @mikecook3]

I've just signed up for a subscription also …thanks.

[Dave MilbournParticipant @davemilbourn48782]

Mike

I would fit the original HP61 F R/C glowplug motor but that's not an option these day.

Whatever you use will need pretty big bucks if it's to replicate the planing performance of the original model. A big fat brushless motor is the obvious way to go but I have no experience yet of using these so couldn't advise you further. Alternatively I have a couple of 3:1 planetary-geared Darke Horse 785 motors from Australia which are very powerful when run on 20 cells. One of those with 2 x 10 packs of NiMH cells and an ACTion P98 or Mycrogyros FR40 ESC would be suitable. You would need to contact me about buying the motor and gearbox and be sitting down when you hear the price…………I'm afraid that £150 wouldn't cover the motor and ESC, never mind the batteries. A cheaper option – if you can find one – would be a Graupner Speed 900 motor, again running on 18 or 20 cells. Be careful what you buy because some of the 900 'clones' are only half the speed of the real item on the same supply voltage.

Whatever you do end up trying DON'T be tempted to fit lead-acid batteries. They may be cheap but they can't deliver high power and they are HEAVY.

You might just be able to obtain a back-copy of the Winter Special from MyTimeMedia – I think it's been withdrawn from W H Smiths' shelves by now.

Dave M

[Mike Cook 3Participant @mikecook3]

Thanks Dave, I appreciate your candour..I shall seriously re – think my budget. I do still have the original Merco 61 so perhaps its a option I might reconsider.

Thanks again for taking the trouble to advise.

Kind regards

[shipwrightParticipant @shipwright]

Hello Mike – just one or two points following on from Dave's postings :

a. "kv" is a simple constant that relates to the emf ("voltage&quot induced in the armature winding and the rpm of the armature or shaft. It should be k "suffix"v but not usually shown as such. So if kv is 1,000 it means that 1 volt is induced in the armature at 1000 rpm, 2 volts at 2,000 rpm etc etc. There is another constant known as the "torque constant" for a motor that relates the current flowing through the armature to the torque ("turning force&quot produced at the shaft. The constants are not independent – if kv is specified then kt can be calculated (kt will usually be given in units of in mNm per amp).

b. The ampere hour capacity of a battery is usually specified at a discharge rate numerically equal to 1/20 th of the AH rating. So if a battery is 5 AH then you will get a duration of 20 hours at a discharge current of 5/20 = 0.25 Amps. But be aware that if you discharge the battery at a higher current the effective AH capacity will be lower.For example at 1 amp discharge current the 5 AH battery should give you 5 hours duration but actually you will get less say only 3 hours.

Hope that helps.

Ian

[Tony HadleyParticipant @tonyhadley]

Mike,

Before giving consideration to re-installing the Merco, check the water you plan to sail on will accept i.c. powered models. If you plan to join a club this again is worth checking. Sailing waters and clubs which now accept i.c. power are few and far between, re-installing the Merco could involve a long journey to sail the model.

Tony

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